Study On Measures To Control Risk And Setup Of Probability Safety Assessment Model For Aferloading Radiotherapy Unit

With the development of computer technology, high dose rate (HDR) afterloading is widely used in tumor therapy, due to its features of high automation and high efficiency etc. Because of high dose rate employed, normally above 12Gy/h, and complicated operating steps involved in therapy process, radiation accident posing harm to both patient and worker can be induced by any minor error. Therefore, it is important to analyse the harzard fator, determine the weak links and put up forward corretcting measures so as to reduce the risk of high dose rate afterloading radiotherapy in field of medical radiation safety.Based on probability theory, probability safety assessment (PSA) is an effective way to control or reduce the risk. The successful application and development of PSA technology in aviation and atomic energy as well as several medical radiation equipments suggest that the feasibility of employment in risk assessment of HDR afterloading radiotherapy.Many a basic data needed by PSA were collected from 23 units nationwide by adopting 4 means of litature survey, on-site investigation, interview and filling the form. Systematic probability safety assessment for HDR afterloading radiotherapy was carried out by analyzing and studying the data collected according to mehods of process tree analysis, hazard analysis and critical control point, fault tree analysis and human reliability analysis. The whole process of HDR afterloading radiotherapy consisits of 4 functions,20 tasks and 177 steps (including 55 steps related to quality control), of which 25 steps are in high risk group of risk value greater than 10, accounting for 14.1% of totality,128 steps are in middle risk group of risk value ranging from 5 to 10, acccouting for 72.3%,24 steps are in low risk group of risk value less than 5, acccouting for 13.6%. Human error probability during emergency responses to source blockage in afterloading rangs from 0.04 to 0.27.Results show that HDR afterloading radiotherapy is safe and reliable on the whole. Further measures such as improving safety by intensifying the monitoring of 9 critical control points and 25 steps with relative high risk and taking correcting measures in time are still strongly recommended for the sake of safety of patient and worker.